Dome-shaped resonator for nuclear magnetic resonance imaging and spectroscopy

ABSTRACT

A radiofrequency resonator for nuclear magnetic resonance imaging and spectroscopy of the human head in which the geometry of the resonator comprises a single end ring connected to a plurality of legs which extend along a cylinder and which are joined in pairs on a hemispherical dome.

BACKGROUND OF THE INVENTION

The present invention relates to a radiofrequency resonator for nuclearmagnetic resonance imaging and spectroscopy of the human head, whosegeometry consists of a single end-ring connected to a plurality of legswhich extend along a cylinder and are joined in pairs on a hemisphericaldome.

Within this application several publications are referenced by arabicnumerals within parentheses. Full citations for these and otherreferences may be found at the end of the specification immediatelypreceding the claims. The disclosures of all of these publications intheir entireties are hereby incorporated by reference into thisapplication in order to more fully describe the state of the art towhich this invention pertains.

For the resonator described above the most homogeneous normal mode ofthis structure is doubly degenerate and affords quadrature operation.The high sensitivity in the hemispherical end is particularly suited tohuman brain studies. This resonator represents a clinical application oftwo-dimensional ladder network resonant structures whose operation maybe understood by analogy to the mechanical problem of oscillatingtwo-dimensional membranes.

Straightforward analysis of the resonant behavior of LC ladder networksmay be accomplished by solving the eigenvalue problem defined by theKirchoff mesh equations. The low-pass birdcage resonator (1) has beencompletely characterized using this technique (2,3). In some cases, ananalogy to one-dimensional mechanical coupled mass-spring systems, wherethe mesh current amplitudes are analogous to the displacements of themasses, provides a more intuitive understanding of these networks (2).For example, the low-pass birdcage resonator is analogous to aone-dimensional coupled mass-spring system with periodic boundaryconditions. The amplitudes of the resulting mesh currents varysinusoidally with the mesh index and integral numbers of wavelengths areallowed (2). Similarly, a nine-leg half-birdcage resonator (4) has beenshown to correspond to a coupled mass-spring system with fixed endconditions which result in sinusoidal distributions of mesh currentamplitudes with half-integral multiples of the wavelength allowed.

Recently, the electro-mechanical analogy has been extended to finitelength, two-dimensional LC ladder networks (5). Specifically, thecorrespondence between a 2D ladder network resonator and a mechanicalvibrating membrane was exploited (6). By assuming independent resonantoperation in each spatial dimension and applying appropriate boundaryconditions, the resonant mode structures of square-mesh, planar 2Dladder networks were predicted (5).

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a cylindrical volumeresonator.

It is a further object of the present invention to provide a cylindricalvolume resonator exhibiting degenerate modes for quadrature operationand a B₁ sensitivity profile which is especially suited for nuclearmagnetic resonance studies of the human head.

According to one aspect of the present invention, a radiofrequencyresonator for nuclear magnetic resonance imaging and spectroscopy of apatient is provided, comprising a hollow cylindrical support structurewith an outer surface, an inner diameter, an open end, and a dome-shapedclosed end, an end ring conductor attached to the outer surface of thehollow cylindrical support structure adjacent the open end, eightsubstantially equal length leg conductors with respective first ends,second ends, and midpoints, each of the respective first ends beingelectrically joined to the end ring conductor at positions spacedsubstantially 45 degrees apart from one another, each of the eight legconductors being attached to the outer surface of the hollow cylindricalsupport structure and each respective second end of the eight legconductors terminating adjacent the dome-shaped closed end, and fourdome conductors attached to the outer surface of the dome-shaped closedend, each of the four dome conductors having respective midpoints andtwo endpoints, an endpoint of each dome conductor being electricallyjoined to respective second ends of pairs of leg conductors spaced135/225 degrees apart from one another, whereby each dome conductor isconnected to two of said leg conductors and whereby each leg conductoris connected to one dome conductor, wherein each of said four domeconductors intersects two of said four dome conductors at twointersection points and wherein each of said four dome conductors thatintersect each of said two dome conductors are electrically joined tosaid two intersecting dome conductors at said intersection points.

According to another aspect of the present invention, a radiofrequencyresonator for nuclear magnetic resonance imaging and spectroscopy of apatient is provided, comprising an end ring conductor with a centralaxis, eight substantially equal length leg conductors with respectivefirst ends, second ends, and midpoints, each of the respective firstends being electrically joined to the end ring conductor at positionsspaced substantially 45 degrees apart from one another, each of theeight leg conductors being substantially parallel with the central axisand each of the respective second ends of the eight leg conductorsterminating on the same side of the end ring conductor, and four domeconductors having respective midpoints and two endpoints, an endpoint ofeach dome conductor being electrically joined to respective second endsof pairs of leg conductors spaced 135/225 degrees apart from oneanother, whereby each dome conductor is connected to two of said legconductors and whereby each leg conductor is connected to one domeconductor, wherein each midpoint of each dome conductor is a respectivepredetermined distance from the end ring conductor, said predetermineddistances being greater than a distance between the first and secondends of the leg conductors, whereby the dome conductors form a domeshape, each of said four dome conductors intersecting two of said fourdome conductors at two intersection points and each of said four domeconductors that intersect each of said two dome conductors beingelectrically joined to said two intersecting dome conductors at saidintersection points.

According to another aspect of the present invention, a radiofrequencyresonator for nuclear magnetic resonance imaging and spectroscopy of apatient is provided, comprising a hollow cylindrical support structurewith an outer surface, an inner diameter, an open end, and a dome-shapedclosed end, an end ring conductor attached to the outer surface of thehollow cylindrical support structure adjacent the open end, sixteensubstantially equal length leg conductors with respective first ends,second ends, and midpoints, each of the respective first ends beingelectrically joined to the end ring conductor at positions spacedsubstantially 22.5 degrees apart from one another, each of the sixteenleg conductors being attached to the outer surface of the hollowcylindrical support structure and each respective second end of thesixteen leg conductors terminating adjacent the dome-shaped closed end,and eight dome conductors attached to the outer surface of thedome-shaped closed end, each of the eight dome conductors havingrespective midpoints and two endpoints, an endpoint of each domeconductor being electrically joined to respective second ends of pairsof leg conductors spaced 112.5/247.5 degrees apart from one another,whereby each dome conductor is connected to two of said leg conductorsand whereby each leg conductor is connected to one dome conductor,wherein each of said eight dome conductors intersects four of said eightdome conductors at four intersection points and wherein each of saideight dome conductors that intersect each of said four dome conductorsare electrically joined to said four intersecting dome conductors atsaid intersection points.

According to another aspect of the present invention, a radiofrequencyresonator for nuclear magnetic resonance imaging and spectroscopy of apatient is provided, comprising: an end ring conductor with a centralaxis, sixteen substantially equal length leg conductors with respectivefirst ends, second ends, and midpoints, each of the respective firstends being electrically joined to the end ring conductor at positionsspaced substantially 22.5 degrees apart from one another, each of thesixteen leg conductors being substantially parallel with the centralaxis and each of the respective second ends of the sixteen legconductors terminating on the same side of the end ring conductor, andeight dome conductors having respective midpoints and two endpoints, anendpoint of each dome conductor being electrically joined to respectivesecond ends of pairs of leg conductors spaced 112.5/247.5 degrees apartfrom one another, whereby each dome conductor is connected to two ofsaid leg conductors and whereby each leg conductor is connected to onedome conductor, wherein each midpoint of each dome conductor is arespective predetermined distance from the end ring conductor, saidpredetermined distances being greater than a distance between the firstand second ends of the leg conductors, whereby the dome conductors forma dome shape, each of said eight dome conductors intersecting four ofsaid eight dome conductors at four intersection points and each of saideight dome conductors that intersect each of said four dome conductorsbeing electrically joined to said four intersecting dome conductors atsaid intersection points.

According to another aspect of the present invention, a radiofrequencyresonator for nuclear magnetic resonance imaging and spectroscopy of apatient is provided, comprising a hollow cylindrical support structurewith an outer surface, an inner diameter, an open end, and a dome-shapedclosed end, an end ring conductor attached to the outer surface of thehollow cylindrical support structure adjacent the open end, n*2substantially equal length leg conductors with respective first ends,second ends, and midpoints, each of the respective first ends beingelectrically joined to the end ring conductor at positions spacedsubstantially 360/(n*2) degrees apart from one another, each of the n*2leg conductors being attached to the outer surface of the hollowcylindrical support structure and each respective second end of the n*2leg conductors terminating adjacent the dome-shaped closed end, and ndome conductors attached to the outer surface of the dome-shaped closedend, each of the n dome conductors having respective midpoints and twoendpoints, an endpoint of each dome conductor being electrically joinedto respective second ends of pairs of leg conductors spaced(360/(n*2))*((n/2)+1 ) degrees apart from one another in onecircumferential direction and (360)-[(360/(n*2))*((n/2)+1)] degreesapart from one another in another circumferential direction, wherebyeach dome conductor is connected to two of said leg conductors andwhereby each leg conductor is connected to one dome conductor, whereineach of said n dome conductors intersects n/2 of said n dome conductorsat n/2 intersection points and wherein each of said n dome conductorsthat intersect each of said n/2 dome conductors are electrically joinedto said n/2 intersecting dome conductors at said intersection points,and wherein n is an integer greater than 3.

These and other advantages will become apparent from the detaileddescription accompanying the claims and attached drawing figures.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a 3×3 mesh dome resonator according toan embodiment of the present invention;

FIG. 2 is a drawing of a 3×3 mesh dome resonator according to anotherembodiment of the present invention;

FIG. 3 is a diagram of a 3×3 square mesh planar resonator;

FIGS. 4(a) and 4(b) are diagrams of predicted mesh current distributionand resultant leg currents in the (0,1) and (1,0) modes of the 3×3 meshplanar resonator;

FIGS. 5(a) and 5(b) are diagrams of expected leg currents in the (0,1)and (1,0) modes of the 3×3 mesh dome resonator;

FIG. 6(a) shows a B₁ contour plot generated from Biot-Savartcalculations of the transverse B₁ fields of a 3×3 mesh dome resonator ofthe instant invention at a longitudinal cross section through the centerof the 3×3 mesh dome coil;

FIG. 6(b) shows a B₁ contour plot generated from Biot-Savartcalculations of the transverse B₁ fields of an eight leg low-passbirdcage coil at a longitudinal cross section through the center of theeight leg birdcage coil;

FIG. 6(c) shows a B₁ contour plot generated from Biot-Savartcalculations of the transverse B₁ fields of a 3×3 mesh dome resonator ofthe instant invention at an axial cross section through the center ofthe 3×3 mesh dome coil representing the average quadrature field;

FIG. 6(d) shows a B₁ contour plot generated from Biot-Savartcalculations of the transverse B₁ fields of an eight leg low-passbirdcage coil at an axial cross section through the birdcage center;FIG. 7 is a chart showing experimentally determined mode structures ofthe planar 3×3 mesh resonator and the 3×3 mesh dome resonator;

FIG. 8 is a graph showing relative 90 degree pulse power (dB) vs. zposition in the 3×3 mesh dome resonator (∘), and the eight-leg birdcage(□);

FIG. 9(a) shows a sagittal slice image of the head of a normal volunteerobtained with the 3×3 mesh dome resonator of the instant invention at anominal transmitter power of 0.22 kW;

FIG. 9(b) shows a sagittal slice image of the head of a normal volunteerobtained with the eight-leg birdcage head coil with nominal power of0.41 kW;

FIG. 9(c) shows a sagittal slice image of the head of a normal volunteerobtained with the eight-leg birdcage head coil with nominal power of0.22 kW;

FIG. 10 is a schematic diagram of a 5×5 mesh dome resonator according toan embodiment of the present invention;

FIG. 11 is a drawing of a 5×5 mesh dome resonator according to anotherembodiment of the present invention;

FIG. 12 is a diagram of a 5×5 square mesh planar resonator;

FIGS. 13(a) and 13(b) are diagrams of predicted mesh currentdistribution and resultant leg currents in the (0,1) and (1,0) modes ofthe 5×5 mesh planar resonator;

FIG. 14(a) shows a B₁ contour plot generated from Biot-Savartcalculations of the transverse B₁ fields of a 5×5 mesh dome resonator ofthe instant invention at an axial cross section through the center ofthe 5×5 mesh dome coil representing the average quadrature field;

FIG. 14(b) shows a B₁ contour plot generated from Biot-Savartcalculations of the transverse B₁ fields of a sixteen leg low-passbirdcage coil at an axial cross section through the birdcage center;

FIG. 15(a) shows a sagittal slice image of the head of a normalvolunteer obtained with the 5×5 mesh dome resonator of the instantinvention; and

FIG. 15(b) shows a sagittal slice image of the head of a normalvolunteer obtained with the sixteen leg birdcage head coil.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

According to one aspect of the present invention, a radiofrequencyresonator for nuclear magnetic resonance imaging and spectroscopy of apatient is provided, comprising a hollow cylindrical support structurewith an outer surface, an inner diameter, an open end, and a dome-shapedclosed end, an end ring conductor attached to the outer surface of thehollow cylindrical support structure adjacent the open end, eightsubstantially equal length leg conductors with respective first ends,second ends, and midpoints, each of the respective first ends beingelectrically joined to the end ring conductor at positions spacedsubstantially 45 degrees apart from one another, each of the eight legconductors being attached to the outer surface of the hollow cylindricalsupport structure and each respective second end of the eight legconductors terminating adjacent the dome-shaped closed end, and fourdome conductors attached to the outer surface of the dome-shaped closedend, each of the four dome conductors having respective midpoints andtwo endpoints, an endpoint of each dome conductor being electricallyjoined to respective second ends of pairs of leg conductors spaced135/225 degrees apart from one another, whereby each dome conductor isconnected to two of said leg conductors and whereby each leg conductoris connected to one dome conductor, wherein each of said four domeconductors intersects two of said four dome conductors at twointersection points and wherein each of said four dome conductors thatintersect each of said two dome conductors are electrically joined tosaid two intersecting dome conductors at said intersection points.

The radiofrequency resonator may further comprise four capacitorselectrically bridging four respective gaps formed in said four domeconductors at respective ones of the midpoints of the four domeconductors and eight capacitors electrically bridging eight respectivegaps formed in said eight leg conductors at respective ones of themidpoints of the eight leg conductors.

The capacitors may be 20.5 picofarad capacitors. The inner diameter ofsaid hollow cylindrical support structure may be sized to permit thesupport structure to fit over the patient's head. The end ringconductor, the eight leg conductors, and the four dome conductors may be1/2" copper tape. The inner diameter may be 27.9 cm and a length of thehollow cylindrical support structure with the dome-shaped end may be27.9 cm.

According to another aspect of the present invention, a radiofrequencyresonator for nuclear magnetic resonance imaging and spectroscopy of apatient is provided, comprising an end ring conductor with a centralaxis, eight substantially equal length leg conductors with respectivefirst ends, second ends, and midpoints, each of the respective firstends being electrically joined to the end ring conductor at positionsspaced substantially 45 degrees apart from one another, each of theeight leg conductors being substantially parallel with the central axisand each of the respective second ends of the eight leg conductorsterminating on the same side of the end ring conductor, and four domeconductors having respective midpoints and two endpoints, an endpoint ofeach dome conductor being electrically joined to respective second endsof pairs of leg conductors spaced 135/225 degrees apart from oneanother, whereby each dome conductor is connected to two of said legconductors and whereby each leg conductor is connected to one domeconductor, wherein each midpoint of each dome conductor is a respectivepredetermined distance from the end ring conductor, said predetermineddistances being greater than a distance between the first and secondends of the leg conductors, whereby the dome conductors form a domeshape, each of said four dome conductors intersecting two of said fourdome conductors at two intersection points and each of said four domeconductors that intersect each of said two dome conductors beingelectrically joined to said two intersecting dome conductors at saidintersection points.

The radiofrequency resonator may further comprise four capacitorselectrically bridging four respective gaps formed in said four domeconductors at respective ones of the midpoints of the four domeconductors, and eight capacitors electrically bridging eight respectivegaps formed in said eight leg conductors at respective ones of themidpoints of the eight leg conductors.

The capacitors may be 20.5 picofarad capacitors. A diameter of the endring conductor may be sized to allow the end ring conductor to fit overthe patient's head.

According to another aspect of the present invention, a radiofrequencyresonator for nuclear magnetic resonance imaging and spectroscopy of apatient is provided, comprising a hollow cylindrical support structurewith an outer surface, an inner diameter, an open end, and a dome-shapedclosed end, an end ring conductor attached to the outer surface of thehollow cylindrical support structure adjacent the open end, sixteensubstantially equal length leg conductors with respective first ends,second ends, and midpoints, each of the respective first ends beingelectrically joined to the end ring conductor at positions spacedsubstantially 22.5 degrees apart from one another, each of the sixteenleg conductors being attached to the outer surface of the hollowcylindrical support structure and each respective second end of thesixteen leg conductors terminating adjacent the dome-shaped closed end,and eight dome conductors attached to the outer surface of thedome-shaped closed end, each of the eight dome conductors havingrespective midpoints and two endpoints, an endpoint of each domeconductor being electrically joined to respective second ends of pairsof leg conductors spaced 112.5/247.5 degrees apart from one another,whereby each dome conductor is connected to two of said leg conductorsand whereby each leg conductor is connected to one dome conductor,wherein each of said eight dome conductors intersects four of said eightdome conductors at four intersection points and wherein each of saideight dome conductors that intersect each of said four dome conductorsare electrically joined to said four intersecting dome conductors atsaid intersection points.

The radiofrequency resonator may further comprise twenty-four capacitorselectrically bridging twenty-four respective gaps formed in said eightdome conductors at midpoints between said intersection points, andsixteen capacitors electrically bridging sixteen respective gaps formedin said sixteen leg conductors at respective ones of the midpoints ofthe sixteen leg conductors.

The capacitors may be 21.4 picofarad capacitors. The inner diameter ofsaid hollow cylindrical support structure may be sized to permit thesupport structure to fit over the patient's head. The end ringconductor, the sixteen leg conductors, and the eight dome conductors maybe 1/2" copper tape. The inner diameter may be 27.9 cm and a length ofsaid hollow cylindrical support structure with the dome-shaped end maybe 27.9 cm.

According to another aspect of the present invention, a radiofrequencyresonator for nuclear magnetic resonance imaging and spectroscopy of apatient is provided, comprising: an end ring conductor with a centralaxis, sixteen substantially equal length leg conductors with respectivefirst ends, second ends, and midpoints, each of the respective firstends being electrically joined to the end ring conductor at positionsspaced substantially 22.5 degrees apart from one another, each of thesixteen leg conductors being substantially parallel with the centralaxis and each of the respective second ends of the sixteen legconductors terminating on the same side of the end ring conductor, andeight dome conductors having respective midpoints and two endpoints, anendpoint of each dome conductor being electrically joined to respectivesecond ends of pairs of leg conductors spaced 112.5/247.5 degrees apartfrom one another, whereby each dome conductor is connected to two ofsaid leg conductors and whereby each leg conductor is connected to onedome conductor, wherein each midpoint of each dome conductor is arespective predetermined distance from the end ring conductor, saidpredetermined distances being greater than a distance between the firstand second ends of the leg conductors, whereby the dome conductors forma dome shape, each of said eight dome conductors intersecting four ofsaid eight dome conductors at four intersection points and each of saideight dome conductors that intersect each of said four dome conductorsbeing electrically joined to said four intersecting dome conductors atsaid intersection points.

The radiofrequency resonator may further comprise twenty-four capacitorselectrically bridging twenty-four respective gaps formed in said eightdome conductors at midpoints between said intersection points, andsixteen capacitors electrically bridging sixteen respective gaps formedin said sixteen leg conductors at respective ones of the midpoints ofthe sixteen leg conductors.

The capacitors may be 21.4 picofarad capacitors. A diameter of the endring conductor may be sized to allow the end ring conductor to fit overthe patient's head.

According to another aspect of the present invention, a radiofrequencyresonator for nuclear magnetic resonance imaging and spectroscopy of apatient is provided, comprising a hollow cylindrical support structurewith an outer surface, an inner diameter, an open end, and a dome-shapedclosed end, an end ring conductor attached to the outer surface of thehollow cylindrical support structure adjacent the open end, n*2substantially equal length leg conductors with respective first ends,second ends, and midpoints, each of the respective first ends beingelectrically joined to the end ring conductor at positions spacedsubstantially 360/(n*2) degrees apart from one another, each of the n*2leg conductors being attached to the outer surface of the hollowcylindrical support structure and each respective second end of the n*2leg conductors terminating adjacent the dome-shaped closed end, and ndome conductors attached to the outer surface of the dome-shaped closedend, each of the n dome conductors having respective midpoints and twoendpoints, an endpoint of each dome conductor being electrically joinedto respective second ends of pairs of leg conductors spaced(360/(n*2))*((n/2)+1 ) degrees apart from one another in onecircumferential direction and (360)-[(360/(n*2))*((n/2)+1)] degreesapart from one another in another circumferential direction, wherebyeach dome conductor is connected to two of said leg conductors andwhereby each leg conductor is connected to one dome conductor, whereineach of said n dome conductors intersects n/2 of said n dome conductorsat n/2 intersection points and wherein each of said n dome conductorsthat intersect each of said n/2 dome conductors are electrically joinedto said n/2 intersecting dome conductors at said intersection points,and wherein n is an integer greater than 3. It should be noted, however,that while the resonator does exhibit an increase in homogeneity as thenumber of legs, and thus the number of meshes in the matrix increases,the parasitic capacitance increases due to the closeness of the legs. A3×3 mesh resonator according to one embodiment of the present inventionis shown schematically in FIG. 1. The 3×3 mesh resonator 1 is comprisedof a 3×3 matrix of meshes formed of end ring conductor 3, eight legconductors 5(a)-5(h), and four dome conductors 7(a)-7(d). It is assumedthat all coil segments have non-zero self-inductance. As is evident fromFIG. 1, the structure is similar to that of an eight leg, low-passbirdcage except that one end-ring has been eliminated, and the legs havebeen routed across the end to form a dome-like geometry. The segmentswhich intersect on the dome are electrically joined.

An understanding of the behavior of the dome resonator of the instantinvention may be gained by examining the behavior of the planar, 3×3square-mesh resonator shown schematically in FIG. 3. The planarresonator is topologically equivalent to the dome resonator, and itsresonant modes have been described elsewhere (5). Briefly, a meshcurrent analysis yields equations with the same formal solutions as avibrating membrane with free-edge boundary conditions (7). Thisfree-edge condition gives rise to a cosinusoidal mesh current amplitudedistribution in each dimension. The resultant mesh current amplitudesare given by: ##EQU1## where m and n are the mesh indices, Ω and Γ arethe indices of the normal modes of oscillation, and M and N are thenumbers of meshes in each dimension. The coefficient A is a function ofthe mode indices and also depends upon the coupling geometry of theresonator to a driving voltage. Any mode of the resonator may bespecified by the mode index pair, (Ω,Γ).

The resonant modes of the planar coil which are of interest are themembers of the degenerate (0,1)(1,0) doublet. The directions of the meshcurrents and the resulting leg currents for the doublet are shown inFIGS. 4(a) and 4(b), where it is assumed that A(1,0)=A(0,1). Theamplitudes of the nonzero mesh currents are equal, and the amplitude ofthe current in each leg segment is obtained by adding the two meshcurrents which share the segment. The topological equivalence betweenthe 3×3 square-mesh planar resonator and the 3×3 mesh dome resonatorsuggests that the mode structures of the two will be similar. If wechoose simply to neglect the effect of the physical distortion of thedome resonator, FIGS. 5(a) and 5(b) give the expected leg currents forthe degenerate (0,1)(1,0) modes. Hence, the possibility exists that the3×3 mesh dome resonator has a pair of degenerate modes which give riseto a pair of orthogonal homogeneous B₁ fields suitable for operation inquadrature. The results of procedures described below demonstrate thatthe 3×3 mesh dome resonator does indeed exhibit homogeneous quadraturemodes which may be employed for head imaging.

A 3×3 mesh resonator according to another embodiment of the presentinvention is shown in FIG. 2. The 3×3 mesh resonator 10 of thisembodiment is formed of adhesive-backed 1/2 "copper tape mounted on aLUCITE frame 12. The LUCITE frame 12 consists of a cylinder section12(a) with an open end 12(b) and a dome-shaped closed end 12(c). Thecopper tape is adhered to the LUCITE frame 12 in such a manner as toform end ring conductor 14, leg conductors 16(a)-16(h), and domeconductors 18(a)-18(d). The eight leg conductors 16(a)-16(h), extendingfrom equally spaced positions on the end ring conductor 14, are eachelectrically connected to the end ring conductor 14 as well as to onedome conductor. That is, as seen in this Fig. pairs of leg conductorsthat are spaced 135/225 degrees apart are electrically connected toopposite ends of the same dome conductor. (The notation 135/225 meansthat conductors are spaced 135 degrees apart along one circumferentialdirection, and 225 degrees apart in the other circumferential direction.This notation is used elsewhere in this application with the samemeaning). Capacitors 20(a)-(l) are electrically connected between gapsformed at the midpoints of each leg conductor 16(a)-16(h) and each domeconductor 18(a)-18(d). The cylindrical portion of the LUCITE frame 12 is19.5 cm in length and 27.9 cm in diameter. The dome-shaped closed end12(c) is 8.4 cm in height and 27.9 cm in diameter. When the capacitors20(a)-(l) are 20.5 picofarad capacitors, the resonator 10 has its lowestresonant mode at approximately 63.9 MHz.

This radiofrequency 3×3 mesh dome-shaped resonator 10 is driven inquadrature through two series tuned inductive loops (not shown) mountedat 90 degrees with respect to one another on the LUCITE frame 12.

A 5×5 mesh resonator according to another embodiment of the presentinvention is shown schematically in FIG. 10. The 5×5 mesh resonator 30is comprised of a 5×5 matrix of meshes formed of end ring conductor 32,sixteen leg conductors 34(a)-34(p), and eight dome conductors36(a)-36(h). It is assumed that all coil segments have non-zeroself-inductance. As is evident from FIG. 10, the structure is similar tothat of an sixteen leg, low-pass birdcage except that one end-ring hasbeen eliminated, and the legs have been routed across the end to form adome-like geometry. The segments which intersect on the dome areelectrically joined.

A 5×5 mesh resonator according to another embodiment of the presentinvention is shown in FIG. 11. The 5×5 mesh resonator 40 of thisembodiment is formed of adhesive-backed 1/2" copper tape mounted on aLUCITE frame 42. The LUCITE frame 42 consists of a cylinder section42(a) with an open end 42(b) and a dome-shaped closed end 42(c). Thecopper tape is adhered to the LUCITE frame 42 in such a manner as toform end ring conductor 44, leg conductors 46(a)-46(p), and domeconductors 48(a)-48(h). The sixteen leg conductors 46(a)-46(p),extending from equally spaced positions on the end ring conductor 44,are each electrically connected to the end ring conductor 44 as well asto one dome conductor. That is, as seen in this Fig. pairs of legconductors that are spaced 112.5/247.5 degrees apart are electricallyconnected to opposite ends of the same dome conductor. Capacitors50(a)-50(an) are electrically connected between gaps formed at themidpoints of each leg conductor 46(a)-46(p) and between gaps formedbetween intersection points of the dome conductors 48(a)-48(h). Thecylindrical portion of the LUCITE frame 42 is 19.5 cm, in length and27.9 cm in diameter. The dome-shaped closed end 12(c) is 8.4 cm, inheight and 27.9 cm in diameter. When the capacitors 50(a)-50(an) are21.4 picofarad capacitors, the resonator 40 has its lowest resonant modeat approximately 63.9 MHz.

This radiofrequency 5×5 mesh dome-shaped resonator 40 is driven inquadrature through two series tuned inductive loops (not shown) mountedat 90 degrees with respect to one another on the LUCITE frame 42.

Experiments conducted using the apparatus of the present invention willnow be described.

MATERIALS AND METHODS

In one experiment to test the resonant operation predicted above, threeRF resonators were constructed: 1) a planar 3×3 mesh of dimensions 12cm×12 cm, 2) a clinical-size 3×3 mesh dome resonator for head imaging,and 3) an eight-leg low-pass birdcage with the same dimensions as theclinical dome coil. Resonators were constructed on 1/8 "thick LUCITEusing 1/2" copper tape (3M, Austin, Tex.) and porcelain chip capacitors(American Technical Ceramics, Huntington Station, N.Y.). The cylindricalportion of the 3×3 mesh dome resonator was 19.5 cm in length and 27.9 cmin diameter. The LUCITE dome was cut from a 12" hemisphere to fit the11" diameter cylinder in an attempt to approximately conform to theshape of the human head. The extent of the arches of the coil was 8.4 cmbringing the total coil length to 27.9 cm. The birdcage resonatordiameter and length were each 27.9 cm.

The expected transverse B₁ fields of the 3×3 mesh dome resonator and thebirdcage were calculated using the Biot-Savart law. Equal currents weresupplied to each coil. In the leg currents mode of the dome resonator,only two current loops exist (see FIGS. 5(a) and 5(b)) and the currentshave equal amplitudes.

Resonant frequencies of the structures were identified using a weaklycoupled inductive loop connected to the impedance bridge accessory of anetwork analyzer (Hewlett Packard model 4195A, Palo Alto, Calif.) whichwas operating in the impedance measurement mode. Qualitatively, thehomogeneity of each of the degenerate (0,1)(1,0) modes of the 3×3 meshdome resonator was ascertained by noting the variation in signal due tochanges in B₁ flux as the pickup loop was moved about within theresonator. The porcelain chip capacitors were used to tune this mode toapproximately 63.9 MHz. The dome and birdcage resonators were drivenusing inductively-coupled series-tuned drive loops positioned overmeshes 90 degrees apart. Fine tuning and mode isolation was accomplishedusing 0-120 pF variable capacitors (Voltronics, Denville, N.J.) mountedon four different legs.

Quadrature operation of the volume resonators was obtained using a 90degree hybrid splitter/combiner (Triangle Microwave, East Hanover,N.J.). Mode isolation was tested on the benchtop by driving each coilthrough the splitter with a sweep generator (Wavetek, Model 1062, SanDiego, Calif.) in continuous wave mode at 63.9 MHz. The flux through aninductive pickup loop was monitored on a 100 MHz oscilloscope(Tektronix, model 2236, Beaverton, Oreg.) to determine the ellipticityof the quadrature field (8) and adjustments were made to the tune andisolation capacitors if necessary. Transmission measurements on thenetwork analyzer demonstrated isolation of greater than 25 dB betweenthe channels when each resonator was loaded with a 3 liter cylindricalphantom comprised of 50 mM saline doped with 5 mM CuSO₄ whichapproximated loading by the human head.

On the clinical scanner (General Electric, Signa 1.5 Tesla, Milwaukee,Wis.), quadrature operation was optimized by adjustment of tune andisolation capacitors to maximize the projected signal from an axialslice through the center of the phantom described above. The final testof quadrature operation was done by reversing the transmit and receivecables to obtain a null image of the phantom (8). The transverse B₁field of each volume coil was indirectly mapped by determining thenecessary transmitter power for a 90 degree flip angle at points alongthe central axis of the coil. Finally, head images were obtained fromvolunteers.

In another experiment to test the resonant operation predicted above,two RF resonators were constructed: 1) a clinical-size 5×5 mesh domeresonator for head imaging, and 2) a sixteen-leg low-pass birdcage withthe same dimensions as the clinical dome coil. Resonators wereconstructed on 1/8 "thick LUCITE material using 1/2" copper tape (3M,Austin, Tex.) and porcelain chip capacitors (American TechnicalCeramics, Huntington Station, N.Y.). The cylindrical portion of the 5×5mesh dome resonator was 19.5 cm in length and 27.9 cm in diameter. TheLUCITE dome was cut from a 12" hemisphere to fit the 11" diametercylinder in an attempt to approximately conform to the shape of thehuman head. The extent of the arches of the coil was 8.4 cm bringing thetotal coil length to 27.9 cm. The birdcage resonator diameter and lengthwere each 27.9 cm.

Resonant frequencies of the structures were identified using a weaklycoupled inductive loop connected to the impedance bridge accessory of anetwork analyzer which was operating in the impedance measurement mode.The 5×5 mesh dome and birdcage resonators were driven usinginductively-coupled series-tuned drive loops positioned over meshes 90degrees apart. Quadrature operation of the volume resonators wasobtained using a 90 degree hybrid splitter/combiner.

On the clinical scanner (General Electric, Signa 1.5 Tesla, Milwaukee,Wis.), quadrature operation was optimized by adjustment of tune andisolation capacitors to maximize the projected signal from an axialslice through the center of a 2 liter, 50 mM NaCl phantom.

The transverse B₁ field of each volume coil was mapped by determiningthe necessary transmitter power for a 90 degree flip angle at pointsalong the central axis of the coil. Finally, head images were obtainedfrom volunteers.

RESULTS

FIGS. 6(a)-6(d) contain the results of the Biot-Savart calculations forthe 3×3 mesh dome resonator and its birdcage counterpart. Identicalscale factors were used in all plots. FIG. 6(a) represents a snapshot intime of the transverse B₁ field in a cross section along the long axisof the 3×3 mesh dome resonator. FIG. 6(b) represents a snapshot in timeof the transverse B₁ field in a cross section along the long axis of thebirdcage resonator. While similar sensitivity was predicted for theresonators at their open ends, the 3×3 mesh dome was expected to haveincreased sensitivity at the closed end. In FIG. 6(c) an axial profileof the average quadrature B₁ field at the longitudinal center of the 3×3mesh dome resonator is presented. In FIG. 6(d) an axial profile of theaverage quadrature B₁ field at the longitudinal center of the birdcageresonator is presented. The four-fold symmetry of the 3×3 mesh domeoperating in quadrature resulted in axial homogeneity comparable to thatof the eight leg birdcage.

The experimentally observed resonant modes of the 3×3 mesh planar anddome resonators are shown in FIG. 7. The mode structures were clearlysimilar and both resonators demonstrated low pass characteristics, i.e.,the lowest frequency mode was the most homogeneous. The frequencyspacings of all modes of the dome resonator except the (2,2) mode werecompressed compared to those of the planar resonator. As indicated bythe asterisk, the (2,2) mode of the dome resonator consisted of currentflowing only in the central mesh at the top of the coil, which was notthe predicted mesh current distribution. The reduction in inductance dueto the limited current path in this mode most likely resulted in thehigh resonant frequency.

A plot of the relative 90 degree pulse power (in dB) vs. z-axis positionfor the 3×3 mesh dome resonator and the eight leg low-pass birdcage isshown in FIG. 8. At the open end (positive z direction), the 90 degreepulse power values of the two coils were similar, indicating that the B₁field strength of the dome coil was quite similar to that of thebirdcage. However, at the closed end there was an increase in B₁intensity of the dome resonator over the birdcage as indicated by thereduction in power necessary for a 90 degree flip angle.

FIG. 9(a) shows a sagittal head image of a 38 year old normal volunteeracquired with the 3×3 dome resonator of the instant invention. FIGS.9(b) and 9(c) show sagittal head images of a 38 year old normalvolunteer acquired with the eight-leg birdcage coil. In each case, thecoil was centered in the field of view and the bridge of the nose waspositioned at the coil center. The spin echo scan parameters were: slicethickness =5 mm, field of view=24 cm, repetition time=500 ms, echotime=11 ms. Gain, window, and level parameters were identical. Thetransmitter power in FIGS. 9(a) and 9(b) was chosen by the autoprescanalgorithm of the scanner to maximize the integrated signal over theslice. An advantage in sensitivity in the dome resonator was visible inthe superior region of the head. The nominal transmitter powercorresponding to a relative power value of 20 dB was 3 kW. In theseimages, the 3×3 mesh dome resonator of this embodiment requiredapproximately 0.22 kW while the birdcage required 0.41 kW to excite thesame sagittal slice. FIG. 9(c) contains the image which resulted whenthe birdcage coil was supplied with a nominal power of 0.22 kW.

FIGS. 14(a) and 14(b) contain the results of the Biot-Savartcalculations for the 5×5 mesh dome resonator and its birdcagecounterpart. Identical scale factors were used in all plots. In FIG.14(a) an axial profile of the average quadrature B₁ field at thelongitudinal center of the 5×5 mesh dome resonator is presented. In FIG.14(b) an axial profile of the average quadrature B₁ field at thelongitudinal center of the birdcage resonator is presented. The 5×5 meshdome operating in quadrature resulted in axial homogeneity comparable tothat of the sixteen leg birdcage.

FIG. 15(a) shows a sagittal head image of a volunteer acquired with the5×5 dome resonator of the instant invention. FIG. 15(b) shows a sagittalhead image of a volunteer acquired with the sixteen leg birdcage coil.The test parameters were: slice thickness=5 mm, repetition time=500 ms,echo time=11 ms. The superior sensitivity in the dome end of the 5×5mesh dome resonator of the instant invention is evident.

DISCUSSION

The experimentally determined mode structure of the 3×3 mesh domeresonator of the instant invention corresponded well to thetheoretically predicted mode structure of the planar 3×3 resonatordespite the physical deformation. This correspondence indicated that anunderstanding of the behavior of the simpler planar geometry may beadequate for predicting the behavior of more complicated 2 D resonant LCnetworks.

The 3×3 and 5×5 mesh dome resonators demonstrate a successful clinicalapplication of two-dimensional ladder network resonators. The highsensitivity in the closed end and good homogeneity predicted by theBio-Savart theory were confirmed by the on-axis power measurements andthe images. The high-quality volunteer head image in FIGS. 9(a) and15(a) show clinical utility for imaging. One possible specificapplication is in functional imaging where SNR is at a premium.

In vivo nuclear magnetic resonance spectroscopy of metabolites alsosuffers from low sensitivity and could benefit from application of theinstant invention. A shortened, quadrature version of a birdcage coilhas been implemented for ³¹ p chemical shift imaging of the human headand has been found to improve the sensitivity of the experiment (9). The3×3 dome resonator length used in the above test was made the same asthat of the standard clinical birdcage coil for purposes of comparison.However, the length of the cylindrical portion of the coil could bereduced to possibly improve the sensitivity in the closed end.

The theory of operation of the structure described above is predicts adegenerate doublet whose homogeneity results in high quality images ofthe human head. The 3×3 and 5×5 mesh dome resonators are thus twoexamples of new coil geometries based upon two-dimensional laddernetwork resonators which have considerable clinical potential.

It must be noted that although the present invention is described byreference to particular embodiments thereof, many changes andmodifications of the invention may become apparent to those skilled inthe art without departing from the spirit and scope of the invention,which is only limited by the appended claims. For example, the resonatorgeometry may be changed to include a matrix of a different number ofmeshes, other than the 3×3 and 5×5 mesh geometries described above.Therefore, the embodiments shown and described are only illustrative,not restrictive.

REFERENCES

1. C. E. Hayes, W. A. Edelstein, J. F. Schenck, O. M. Mueller, M.Eash,J. Magn. Reson. 63, 622-628 (1985).

2. J. Tropp, J. Magn. Reson. 83, 51-62 (1989).

3. P. M. Joseph, D. Lu, IEEE Trans. Med. Imag. 8(3), 286-294 (1989).

4. D. Ballon, M. C. Graham, S. Miodownik, J. A. Koutcher, J. Magn.Reson. 90, 131-140 (1990).

5. D. Ballon, K. L. Meyer, proceedings of the Society of MagneticResonance in Medicine, Twelfth Annual Meeting, New York, p. 1323 (1993).

6. A. P. French, "Vibration and Waves," 181-188, Thomas Nelson and Sons,Ltd. London, 1971.

7. I. G. Main, "Vibrations and Waves in Physics," Third Edition,158-159, Cambridge University Press, Cambridge, 1993.

8. G. H. Glover, C. E. Hayes, N. J. Pelc, W. A. Edelstein, O. M.Mueller, H. R. Hart, C. J. Hardy, M. O'Donnell, W. D. Barber, J. Magn.Reson. 64, 255-270 (1985).

9. P. A. Bottomley, H. C. Charles, P. B. Roemer, D. Flamig, H. Engeseth,W. A. Edelstein, O. M. Mueller, Magn. Reson. Med. 7,319-336, (1988).

What is claimed is:
 1. A radiofrequency resonator for nuclear magnetic resonance imaging and spectroscopy of a patient, comprising:a hollow cylindrical support structure with an outer surface, an inner diameter, an open end, and a dome-shaped closed end; an end ring conductor attached to the outer surface of the hollow cylindrical support structure adjacent the open end; eight substantially equal length leg conductors with respective first ends, second ends, and midpoints, each of the respective first ends being electrically joined to the end ring conductor at positions spaced substantially 45 degrees apart from one another, each of the eight leg conductors being attached to the outer surface of the hollow cylindrical support structure and each respective second end of the eight leg conductors terminating adjacent the dome-shaped closed end; and four dome conductors attached to the outer surface of the dome-shaped closed end, each of the four dome conductors having respective midpoints and two endpoints, an endpoint of each dome conductor being electrically joined to respective second ends of pairs of leg conductors spaced 135/225 degrees apart from one another, whereby each dome conductor is connected to two of said leg conductors and whereby each leg conductor is connected to one dome conductor; wherein each of said four dome conductors intersects two of said four dome conductors at two intersection points and wherein each of said four dome conductors that intersect each of said two dome conductors are electrically joined to said two intersecting dome conductors at said intersection points.
 2. The radiofrequency resonator according to claim 1, further comprising:four capacitors electrically bridging four respective gaps formed in said four dome conductors at respective ones of the midpoints of the four dome conductors; and eight capacitors electrically bridging eight respective gaps formed in said eight leg conductors at respective ones of the midpoints of the eight leg conductors.
 3. The radiofrequency resonator according to claim 2, wherein said capacitors are 20.5 picofarad capacitors.
 4. The radiofrequency resonator according to claim 2, wherein said inner diameter of said hollow cylindrical support structure is sized to permit the support structure to fit over the patient's head.
 5. The radiofrequency resonator according to claim 1, wherein said end ring conductor, said eight leg conductors, and said four dome conductors are 1/2" copper tape.
 6. The radiofrequency resonator according to claim 1, wherein said inner diameter is about 27.9 cm and a length of said hollow cylindrical support structure with the dome-shaped end is about 27.9 cm.
 7. A radiofrequency resonator for nuclear magnetic resonance imaging and spectroscopy of a patient, comprising:an end ring conductor with a central axis; eight substantially equal length leg conductors with respective first ends, second ends, and midpoints, each of the respective first ends being electrically joined to the end ring conductor at positions spaced substantially 45 degrees apart from one another, each of the eight leg conductors being substantially parallel with the central axis and each of the respective second ends of the eight leg conductors terminating on the same side of the end ring conductor; and four dome conductors having respective midpoints and two endpoints, an endpoint of each dome conductor being electrically joined to respective second ends of pairs of leg conductors spaced 135/225 degrees apart from one another, whereby each dome conductor is connected to two of said leg conductors and whereby each leg conductor is connected to one dome conductor; wherein each midpoint of each dome conductor is a respective predetermined distance from the end ring conductor, said predetermined distances being greater than a distance between the first and second ends of the leg conductors, whereby the dome conductors form a dome shape, each of said four dome conductors intersecting two of said four dome conductors at two intersection points and each of said four dome conductors that intersect each of said two dome conductors being electrically joined to said two intersecting dome conductors at said intersection points.
 8. The radiofrequency resonator according to claim 7, further comprising:four capacitors electrically bridging four respective gaps formed in said four dome conductors at respective ones of the midpoints of the four dome conductors; and eight capacitors electrically bridging eight respective gaps formed in said eight leg conductors at respective ones of the midpoints of the eight leg conductors.
 9. The radiofrequency resonator according to claim 8, wherein said capacitors are 20.5 picofarad capacitors.
 10. The radiofrequency resonator according to claim 7, wherein a diameter of the end ring conductor is sized to allow the end ring conductor to fit over the patient's head.
 11. A radiofrequency resonator for nuclear magnetic resonance imaging and spectroscopy of a patient, comprising:a hollow cylindrical support structure with an outer surface, an inner diameter, an open end, and a dome-shaped closed end; an end ring conductor attached to the outer surface of the hollow cylindrical support structure adjacent the open end; sixteen substantially equal length leg conductors with respective first ends, second ends, and midpoints, each of the respective first ends being electrically joined to the end ring conductor at positions spaced substantially 22.5 degrees apart from one another, each of the sixteen leg conductors being attached to the outer surface of the hollow cylindrical support structure and each respective second end of the sixteen leg conductors terminating adjacent the dome-shaped closed end; and eight dome conductors attached to the outer surface of the dome-shaped closed end, each of the eight dome conductors having respective midpoints and two endpoints, an endpoint of each dome conductor being electrically joined to respective second ends of pairs of leg conductors spaced 112.5/247.5 degrees apart from one another, whereby each dome conductor is connected to two of said leg conductors and whereby each leg conductor is connected to one dome conductor; wherein each of said eight dome conductors intersects four of said eight dome conductors at four intersection points and wherein each of said eight dome conductors that intersect each of said four dome conductors are electrically joined to said four intersecting dome conductors at said intersection points.
 12. The radiofrequency resonator according to claim 11, further comprising:twenty-four capacitors electrically bridging twenty-four respective gaps formed in said eight dome conductors at midpoints between said intersection points; and sixteen capacitors electrically bridging sixteen respective gaps formed in said sixteen leg conductors at respective ones of the midpoints of the sixteen leg conductors.
 13. The radiofrequency resonator according to claim 12, wherein said capacitors are 21.4 picofarad capacitors.
 14. The radiofrequency resonator according to claim 12, wherein said inner diameter of said hollow cylindrical support structure is sized to permit the support structure to fit over the patient's head.
 15. The radiofrequency resonator according to claim 11, wherein said end ring conductor, said sixteen leg conductors, and said eight dome conductors are 1/2" copper tape.
 16. The radiofrequency resonator according to claim 11, wherein said inner diameter is about 27.9 cm and a length of said hollow cylindrical support structure with the dome-shaped end is about 27.9 cm.
 17. A radiofrequency resonator for nuclear magnetic resonance imaging and spectroscopy of a patient, comprising:an end ring conductor with a central axis; sixteen substantially equal length leg conductors with respective first ends, second ends, and midpoints, each of the respective first ends being electrically joined to the end ring conductor at positions spaced substantially 22.5 degrees apart from one another, each of the sixteen leg conductors being substantially parallel with the central axis and each of the respective second ends of the sixteen leg conductors terminating on the same side of the end ring conductor; and eight dome conductors having respective midpoints and two endpoints, an endpoint of each dome conductor being electrically joined to respective second ends of pairs of leg conductors spaced 112.5/247.5 degrees apart from one another, whereby each dome conductor is connected to two of said leg conductors and whereby each leg conductor is connected to one dome conductor; wherein each midpoint of each dome conductor is a respective predetermined distance from the end ring conductor, said predetermined distances being greater than a distance between the first and second ends of the leg conductors, whereby the dome conductors form a dome shape, each of said eight dome conductors intersecting four of said eight dome conductors at four intersection points and each of said eight dome conductors that intersect each of said four dome conductors being electrically joined to said four intersecting dome conductors at said intersection points.
 18. The radiofrequency resonator according to claim 17, further comprising:twenty-four capacitors electrically bridging twenty-four respective gaps formed in said eight dome conductors at midpoints between said intersection points; and sixteen capacitors electrically bridging sixteen respective gaps formed in said sixteen leg conductors at respective ones of the midpoints of the sixteen leg conductors.
 19. The radiofrequency resonator according to claim 18, wherein said capacitors are 21.4 picofarad capacitors.
 20. The radiofrequency resonator according to claim 17, wherein a diameter of the end ring conductor is sized to allow the end ring conductor to fit over the patient's head.
 21. A radiofrequency resonator for nuclear magnetic resonance imaging and spectroscopy of a patient, comprising:a hollow cylindrical support structure with an outer surface, an inner diameter, an open end, and a dome-shaped closed end; an end ring conductor attached to the outer surface of the hollow cylindrical support structure adjacent the open end; n*2 substantially equal length leg conductors with respective first ends, second ends, and midpoints, each of the respective first ends being electrically joined to the end ring conductor at positions spaced substantially 360/(n*2) degrees apart from one another, each of the n*2 leg conductors being attached to the outer surface of the hollow cylindrical support structure and each respective second end of the n*2 leg conductors terminating adjacent the dome-shaped closed end; and n dome conductors attached to the outer surface of the dome-shaped closed end, each of the n dome conductors having respective midpoints and two endpoints, an endpoint of each dome conductor being electrically joined to respective second ends of pairs of leg conductors spaced (360/(n*2))*((n/2)+1) degrees apart from one another in one circumferential direction and (360) -[(360/(n*2))*((n/2)+1)] degrees apart from one another in another circumferential direction, whereby each dome conductor is connected to two of said leg conductors and whereby each leg conductor is connected to one dome conductor; wherein each of said n dome conductors intersects n/2 of said n dome conductors at n/2 intersection points and wherein each of said n dome conductors that intersect each of said n/2 dome conductors are electrically joined to said n/2 intersecting dome conductors at said intersection points, and wherein n is an integer greater than
 3. 